1. Field of the Invention
The present invention relates to an apparatus which forms a thin film on a substrate and a method for forming a thin film using the apparatus. For example, when a film is formed on a glass substrate using a sputtering apparatus and the like, on the occasion that sputtering grains deposit at desired positions on the substrate to form a thin film, such a thin film tends to be formed, so that the distribution of film thickness gives a peak in a portion of the substrate corresponding to a target center in the radial direction of the rotatable substrate in spite of rotation of the substrate intended to allow film formation conditions to be uniform. Furthermore, in the circumferential direction of the rotatable substrate, depending on the places where film formation is started and ended on the rotated substrate, such a distribution of film thickness tends to be obtained that these places constitute the start and end points of the distribution. The dispersion of such a film thickness tends to be several percent out of a desired film thickness value. However, in the field of optical thin films for use in optical devices, optical filters and the like, it is desirable to form a thin film having a strictly precise and uniform thickness in order to control optical film thickness (film thickness×refractive index), which varies depending on the film thickness.
2. Description of the Related Art
A conventional sputtering apparatus, which rotates a substrate in order to unify film formation conditions and which forms a thin film on this substrate, is constructed as shown in FIG. 1. In this apparatus, a substrate holder 3 rotatably supported by a rotating shaft 2 is provided in the upper portion of an apparatus chamber 1. A glass substrate 4 is mounted on the holder 3. Furthermore, the apparatus chamber 1 has a sputtering cathode 6 having a Ti target 5 facing the substrate 4 arranged at the lower portion of a cross section in one side region thereof, as a film forming source. A protective cover 7 is installed outside a sputtering target composed of the Ti target 5 and the sputtering cathode 6. Furthermore, a shutter 8 having a circular opening 8a is provided in the lower portion of the apparatus chamber 1 and the shutter 8 is supported by a rotating shaft 9 so as to be rotatable around it (see FIG. 2).
In the sputtering apparatus in FIG. 1, the rotating shaft 2 of the substrate holder 3 and the rotating shaft 9 of the shutter 8 can be rotated at each rotation rate independently. Furthermore, the substrate holder 3 and the substrate 4 have a film thickness monitor 10 provided thereon to measure the thickness of a thin film formed on the substrate 4. The film thickness monitor 10 is composed of light emitting sections 10a1 to 10a3 and light receiving sections 10b1 to 10b3 corresponding to the light emitting sections 10a1 to 10a3, each by each. Combinations of the light emitting sections 10a and the light receiving sections 10b comprise the first monitor 10a1–10b1, the second monitor 10a2–10b2 and the third monitor 10a3–10b3. Thus, the optical sensors composed of the light emitting sections 10a1 to 10a3 and the light receiving sections 10b1 to 10b3 constitute a series of monitors (the first to the third monitors), thereby enabling the film thickness monitor 10 to measure the transmittance between the glass substrate 4 and the thin film to monitor the uniformity of the thickness of the thin film. Furthermore, the apparatus chamber 1 can be evacuated by a vacuum pump 11. Furthermore, a gas introducing port 12a is provided in a sputtering target-side region at the lower portion of the cross section of the apparatus chamber 1 so as to introduce sputtering gas therethrough. A gas introducing port 12b is located close to the substrate holder 3 in the upper portion of the cross section of the apparatus chamber 1 so as to introduce reactive gas therethrough.
To form a film on the glass substrate 4, the inside of the chamber 1 is first evacuated as a pre-treatment by the vacuum pump 11. Then, Ar gas is introduced through the gas introducing port 12a as sputtering gas. The shutter 8 is then rotated around the rotating shaft 9 to adjust the opening 8a to the position except for over the target 5. Then, by a presputtering to apply electric power to the sputtering cathode 6, the surface of the target 5 becomes cleaned-up. Subsequently, Ar gas is introduced through the gas introducing port 12a as sputtering gas, while oxygen gas is introduced through the gas introducing port 12b as reactive gas. Furthermore, the shutter 8 is rotated around the rotating shaft 9 to adjust the opening 8a to the position over the target 5. Electric power is applied to the sputtering cathode 6 to sputter the Ti target 5 on the sputtering cathode 6. Thus, an oxide film, TiO2, is formed on the substrate 4. At that time, the substrate holder 3 and thus the substrate 4 are rotating around the rotating shaft 2. Then, TiO2 on the substrate 4 is formed continuously for a predetermined time, while the film thickness monitor 10 is used to measure the thickness of a thin film formed on the substrate 4. Once the thin film has been formed to reach to a predetermined thickness, the shutter 8 is rotated again to adjust the opening 8a to the position except for over the target 5. Then the film formation is finished.
In this conventional apparatus, the shutter 8 is used as means for switching the start and the end of film formation or as means for preventing a target substance from flying to the substrate 4 during the presputtering step. And the shutter 8 also has a function of correcting the distribution of the thickness of a thin film on the substrate 4 by means of the shape of the opening 8a thereof. Japanese Patent Laid-Open No. H4-173972 discloses, in its FIG. 5, a sputtering apparatus comprising a shutter (film thickness correcting plate) having an opening shaped to enable the film thickness to be corrected in the above-mentioned manner in which the shutter (film thickness correcting plate) has the opening 8a. 
However, with a shutter (film thickness correcting plate) having an opening with a fixed shape, it is difficult to take care of changes in various sputtering conditions during sputtering step (the vacuum degree, the amount of gas introduced, the amount of gas released from the chamber, the sputtering voltage, the sputtering current and the like). In particular, it is known in the field of optical thin films, that thin films, such as oxide or nitride films tend to be formed using a reactive sputtering apparatus and that film formation rate and film quality of this case depend on the state of the surface of the target. And the state of the surface of the target is related to the partial pressure of the reactive gas. Generally, the film formation rate and the partial pressure of the reactive gas have such a correlation as shown with a hysteresis curve. Furthermore, the hysteresis curve changes markedly at the time of input electric power, resulting in an unstable state. Consequently, the above-mentioned sputtering conditions tend to be varied.
Thus, Japanese Patent Laid-Open No. S61-183464 discloses, in its FIG. 2, an apparatus in which a large number of film thickness correcting plates movable constitute a film thickness correcting member to adjust the shape of the opening, thereby taking care of a change in distribution of the film thickness. However, this apparatus may fail in maintaining a vacuum degree in the chamber when a driving work for the film thickness correcting plates is carried out. Consequently, this apparatus cannot be efficient from the handling point of view.
Furthermore, the above-mentioned conventional arts disclosed in Japanese Patent Laid-Open Nos. H4-173972 and S61-183464 correct the distribution of the thickness of a thin film formed on the rotatable substrate in the radial direction thereof. Their effects are not sure in correcting the distribution of the film thickness in a circumferential direction, given at the start or end of rotation.
In view of the above-mentioned problems, it is an object of the present invention to provide a thin film forming apparatus which is capable of efficiently correcting the film thickness so as to take care of changes in radial distribution of the film thickness caused in various sputtering conditions and to take care of the circumferential distribution of the film thickness, as well as a method for forming a thin film using this film forming apparatus.